The present invention relates to a method of producing an electrically conductive plastic material in which soot (lamp black) or graphite is used as the so-called conductive pigment.
Pure plastic materials are generally electrically nonconductive. When one talks of a material being conductive, it is generally assumed that the material has a passage resistance of less than 10.sup.4 ohm.cm. Plastic materials are generally made electrically conductive by the incorporation therein of so-called conducting pigments such as specific metallic powders, graphite powder, highly structured lamp black and carbon fibers in a thermoplastic material. More recently, it has been proposed to utilise metallic "whiskers" to increase conductivity.
In general, the conductivity of the material is improved by increasing the proportion of the conducting pigment(s) in the plastic material, a greater proportion of conducting pigments being utilised if it is required to produce a higher degree of conductivity. However, if the proportion of conducting pigments exceeds a certain limit, the plastic material and conducting pigment mixture becomes brittle, and cannot be processed.
It is also known that to obtain a high conductivity, the main criterion is not so much the absolute proportion of conducting pigment in the mixture but rather the capacity of the conducting particles to link or be linked, together to form a cohesive structure, a so-called quasi-metal skeleton, which permits the passage of an electrical current. In such a case, it is important that the cohesive particle structure is not penetrated by the plastic material itself thereby insulating the current-carrying particles from one another.
In most known methods of manufacturing electrically conductive plastic materials, the conducting pigments are worked into the plastic material in a dry state. In order to disperse the particles in the viscous plastic material, high shearing forces must be supplied to the plastic material. The conductivity obtained is dependent, to a high degree, upon the time taken for such mixing because a short mixing time often leads to insufficient dispersion of the conducting pigments in the plastic material whilst long periods of homegenisation leads to the disintegration of the structure.